During the combustion of a fuel, such as coal, oil, peat, waste, etc., in a combustion plant, such as a power plant, a hot process gas is generated, and such hot process gas contains, among other components, nitrogen oxides, usually denoted as NOx, and sulphur oxides, usually denoted as SOx. The nitrogen oxides are mainly comprised of nitrogen monoxide, denoted as NO, and nitrogen dioxide, denoted as NO2, and the sulphur dioxides are mainly comprised of sulphur dioxide, denoted as SO2, and sulphur trioxide, denoted as SO3, the amount of sulphur trioxide, SO3, normally constituting less than 5% of the total amount of SOx. The removal of NOx is usually accomplished through the use of a so-called Selective Catalytic Reduction (SCR) process. In accordance with such a process, NOx is reduced by means of ammonia gas, NH3, to form nitrogen gas, N2, in the presence of a catalytically active material. Such a catalytically active material commonly comprises metal oxides, such as, by way of exemplification, vanadium pentoxide, denoted as V2O5, and tungsten trioxide, denoted as WO3.
A problem associated with the use of many of such catalytically active materials is that they after a period of time become contaminated by deposits of sulphur trioxide, SO3, thereon that tend to form ammonium sulphates, which results in a decrease in the nitrogen oxide removal efficiency of these catalytically active materials. The normal procedure that is employed for purposes of overcoming such problems is to operate a reactor containing such catalytically active material at a rather high temperature, usually above 300° C., in an attempt to prevent the sulphur trioxide, SO3, from precipitating out on to the catalytically active material.
In U.S. Pat. No. 5,762,885 there is described an oxidation catalyst absorber, which embodies a number of catalyst segments arranged in parallel relation and comprising platinum or palladium. Each of these catalyst segments is provided with two louvers, such that each one of the respective catalyst segments can be “closed” off insofar as the process gas that is to be cleaned is concerned. Moreover, the catalyst segments that are closed off can be regenerated through the use of a regenerating gas that contains hydrogen gas, the latter being operative to remove pollutants from the catalytic material embodied by such catalyst segments that are closed off. After being spent, such regenerating gas is then recycled to the reactor, where the spent regenerating gas is mixed with the process gas at a point located upstream of the catalyst bed.
While a catalyst absorber such as that described in U.S. Pat. No. 5,762,885 might be deemed to be effective for purposes of effecting the cleaning of a process gas that is generated in a natural gas fired turbine power plant, wherein the concentration of SOx in the process gas is very low, i.e., generally lower than 1 ppm, such a catalyst absorber is not suitable for cleaning process gas, e.g., flue gases, in which the concentration of SOx is higher than about 5 ppm. In a process gas that is generated during the combustion of coal or oil, or during the incineration of waste, the concentration of SOx in such a process gas is often in the range of 10 to 5000 ppm. Furthermore, the type of catalyst, which is employed, that is, an oxidation catalyst that contains noble metals like platinum or palladium, necessitates that there only be an extremely low concentration of catalyst pollutants, such as, by way of exemplification, mercury, lead and other heavy metals, in the hot process gas. Such a requirement restricts the use of the reactor to which U.S. Pat. No. 5,762,885 is directed to so-called “clean fuels”, such as natural gas. For plants in which coal, oil, peat, waste, etc, are combusted the reactor to which U.S. Pat. No. 5,762,885 is directed would not be capable of providing an acceptable level of nitrogen oxide removal efficiency.